The Franck-Condon principle is a fundamental concept in molecular spectroscopy that pertains to the behavior of molecules during electronic transitions. It provides insight into why certain electronic transitions are favored over others in terms of their ability to absorb or emit light. The principle is based on the idea that during an electronic transition, such as the absorption or emission of a photon, the nuclei of the molecules do not have time to move significantly due to the very short timescales involved in electronic transitions.
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Electronic excitation occurs rapidly compared with nuclear motion. In the Condon approximation, vibronic intensities are proportional to squared overlaps of initial and final vibrational wavefunctions; vertical arrows on a potential diagram summarize this overlap rather than a nuclear trajectory.